Fig. 4: Molecular structure of Cu(x)Ca(1-x)O MMO catalysts.
A Ex situ Raman spectra (785 nm laser) of bulk reference compounds including Ca(OH)2 and CuO and Cu0.05Ca0.95O MMO catalyst. B Ex situ Raman spectra (785 nm laser) of Cu(x)Ca(1-x)O catalysts as a function of Cu-content from 5 to 18% Cu. Bands marked with (*) represent vibrational modes of CuO and Ca(OH)2 related domains present in the CuCaO MMO catalysts. C In situ Raman spectra evolution of Cu0.05Ca0.95O catalyst during reaction with 1,4-BDO at 25°C, 150 °C under inert (i.e., reduction half cycle) and under air (i.e., oxidation half cycle). The gray shaded spectrum in each panel is the reference Raman spectrum of as-synthesized Cu0.05Ca0.95O. The as-synthesized spectrum is presented in each panel to draw the eye to the Raman bands assigned to Cu-O-Ca interfacial bonds (500–710 cm−1) that diminish in intensity upon reduction of the catalyst and then reappear upon reoxidation of the catalyst.
